If you ask people who actually go caving, spelunking is the derogatory term for stupid or unprepared cave trips. Origin: "spelunk" is the sound a clumsy caver makes when he slips and falls in a cave and lands in water.

If you don't take a helmet, more than one source of light, and a few friends into the cave with you, then you're not caving. You're spelunking.

When a girl is naked and asleep under the covers and you get a flashlight (or a cell phone will do) and check out the goods. Usually done when you first sleep with someone, but can be done with girlfriends, friends, whatever.
A girl can perform this on a sleeping dude as well.

Laying there next to his hot one night stand, the perv grabs his cell phone and goes spelunking to get a closer look at her love clam.

The act of engaging in sexual relations with a woman who has a large, dark and cavernous vagina. This act of extreme desperation and bravery is usually reserved for men who have gone more than three weeks without getting laid, and who are willing to submerge themselves (without safety equipment and/or flashlights) without preparation into a vagina in which many enter and few return.

Examples:

"Koumi was one of the ballsiest spelunkers I have ever seen: he used to fuck XXXXXX without a condom or a GPS system"

"Guys, we lost one of our own today. I just got word that Andrew Tha Owner" died on a spelunking trip yesterday. He was in Wunsch's Cave when he sprained his ankle, and was unable to escape. He eventually ran out of air, and died."

Having sexual intercourse with a girl who has a very wide and stretched out vagina. The vagina is very cave-like due to its constant past use. If you are considered to be spelunking, then the girl is a whore, and you will most likely contract a sexually transmitted disease.